Beverage preparation machines are well known in the food science and consumer goods area. Such machines allow a consumer to prepare at home a given type of beverage, for instance a coffee-based beverage, e.g. an espresso or a brew-like coffee cup.
Today, most beverage preparation machines for in-home beverage preparation comprise a system made of a machine which can accommodate portioned ingredients for the preparation of the beverage. Such portions can be soft pods or pads, or sachets, but more and more systems use semi-rigid or rigid portions such as rigid pods or capsules. In the following, it will be considered that the beverage machine is a beverage preparation machine working with a rigid or semi-rigid capsule.
The machine preferably comprises a receptacle for accommodating said capsule and a fluid injection system for injecting a fluid, preferably water, under pressure into said capsule. Water injected under pressure in the capsule, for the preparation of a coffee beverage, is preferably hot, that is to say at a temperature above 70° C. However, in some particular instances, it might also be at ambient temperature, or even at a chilled temperature. The pressure inside the capsule chamber during extraction and/or dissolution of the capsule contents is typically about 1 to about 8 bar for dissolution products and about 2 to about 12 bar for extraction of roast and ground coffee. The present invention could also encompass the so-called “brewing” process of beverage preparation—particularly for tea and coffee. Brewing involves a time of infusion of the ingredient by a fluid (e.g. hot water), whereas the extraction or dissolution preparation process allows a consumer to prepare a beverage, for instance coffee, within a few seconds.
Generally, in the following specification, the term “brewing” of an ingredient by a fluid, is meant to encompass extraction of a powdered edible material such as for instance roast and ground powdered coffee, or dissolution of edible soluble material such as for instance soluble tea or coffee, or infusion of an edible material with an infusion fluid under very low relative pressure, or atmospheric pressure, for a longer time than that required for extraction or dissolution, for instance infusion of tea leaves by hot water.
The principle of extracting and/or dissolving the contents of a closed capsule under pressure is known and consists typically of confining the capsule in a receptacle of a machine, injecting a quantity of pressurized water into the capsule, generally after piercing a face of the capsule with a piercing injection element such as a fluid injection needle mounted on the machine, so as to create a pressurized environment inside the capsule either to extract the substance or dissolve it, and then release the extracted substance or the dissolved substance through the capsule. Capsules allowing the application of this principle have already been described for example in applicant's European patent n° EP 1 472 156 B1, and in EP 1 784 344 B1.
Machines allowing the application of this principle have already been described for example in patents CH 605 293 and EP 242 556. According to these documents, the machine comprises a receptacle for the capsule and a perforation and injection element made in the form of a hollow needle comprising in its distal region one or more liquid injection orifices. The needle has a dual function in that it opens the top portion of the capsule on the one hand, and that it forms the water inlet channel into the capsule on the other hand.
The machine further comprises a fluid tank—in most cases this fluid is water—for storing the fluid that is used to dissolve and/or infuse and/or extract under pressure the ingredient(s) contained in the capsule. The machine comprises a heating unit such as a boiler or a heat exchanger, which is able to warm up the water used therein to working temperatures (classically temperatures up to 80-90° C.). Finally, the machine comprises a pump element for circulating the water from the tank to the capsule, optionally though the heating unit. The way the water circulates within the machine is e.g. selected via a selecting valve means, such as for instance a peristaltic valve of the type described in applicant's European patent application EP 2162653 A1.
When the beverage to be prepared is coffee, one interesting way to prepare the coffee is to provide the consumer with a capsule containing roast and ground coffee powder, which is to be extracted with hot water injected therein.
Capsules have been developed for such an application, which are described and claimed in applicant's European patent EP 1 784 344 B1, or in European patent application EP 2 062 831.
In short, such capsules typically comprise:                a hollow body and an injection wall which is impermeable to liquids and to air and which is attached to the body and adapted to be punctured by e.g. an injection needle of the machine,        a chamber containing a bed of roast and ground coffee to be extracted,        an aluminum membrane disposed at the bottom end of the capsule, closing the capsule, for retaining the internal pressure in the chamber, the membrane being associated with piercing means for piercing dispensing holes in the aluminum membrane when the internal pressure inside the chamber reaches a certain pre-determined value,        optionally, means configured to break the jet of fluid so as to reduce the speed of the jet of fluid injected into the capsule and distribute the fluid across the bed of substance at a reduced speed. It is often important for the user to know when the water level in the machine tank is too low to prepare a full beverage.        
As mentioned, such and other beverage preparation machines comprise a heating unit like a boiler or a heat exchanger. One commonly used heating unit in prior art beverage preparation machines is a so-called thermo block where water is heated passing through a number of (tiny) tubes. Thermo blocks can be made of different material like aluminum, steel, copper, etc. However, a disadvantage of such heating units is that the amount of material necessary is relatively large resulting in significant heating losses during heating and furthermore also takes a long time to heat up.
Another commonly used heating unit is the traditional boiler unit. However, that works with mixing hot and cold water together and thereby has the risk of providing cold water or water not hot enough at its outlet, which may be especially detrimental in relation to preparing beverages e.g. like coffee-based beverages as it may greatly impact the taste of the beverage.
It is therefore an objective of the present invention to provide a heating unit that avoids or at least reduces the above-mentioned drawbacks of traditional heating units.